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The Endoskeletal Origin of the Turtle Carapace

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The Endoskeletal Origin of the Turtle Carapace ARTICLE Received 7 Dec 2012 | Accepted 3 Jun 2013 | Published 9 Jul 2013 DOI: 10.1038/ncomms3107 OPEN The endoskeletal origin of the turtle carapace Tatsuya Hirasawa1, Hiroshi Nagashima2 & Shigeru Kuratani1 The turtle body plan, with its solid shell, deviates radically from those of other tetrapods. The dorsal part of the turtle shell, or the carapace, consists mainly of costal and neural bony plates, which are continuous with the underlying thoracic ribs and vertebrae, respectively. Because of their superficial position, the evolutionary origins of these costo-neural elements have long remained elusive. Here we show, through comparative morphological and embryological analyses, that the major part of the carapace is derived purely from endos- keletal ribs. We examine turtle embryos and find that the costal and neural plates develop not within the dermis, but within deeper connective tissue where the rib and intercostal muscle anlagen develop. We also examine the fossils of an outgroup of turtles to confirm that the structure equivalent to the turtle carapace developed independently of the true osteoderm. Our results highlight the hitherto unravelled evolutionary course of the turtle shell. 1 Laboratory for Evolutionary Morphology, RIKEN Center for Developmental Biology, Kobe 650-0047, Japan. 2 Division of Gross Anatomy and Morphogenesis, Department of Regenerative and Transplant Medicine, Niigata University, Niigata 951-8510, Japan. Correspondence and requests for materials should be addressed to T.H. (email: [email protected]). NATURE COMMUNICATIONS | 4:2107 | DOI: 10.1038/ncomms3107 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3107 wo types of skeletal systems are recognized in vertebrates, exoskeletal components into the costal and neural plates (Fig. 1, the exoskeleton composed of the dermal bones and the Supplementary Fig. S1) have been proposed4–20, although the Tcartilaginously preformed endoskeleton1–3. For the past exoskeletal origin of the accessory bones that surround the costo- 200 years, the origin of the turtle carapace has remained unclear, neural plates marginally, namely the nuchal, peripheral, and several different hypotheses about incorporation of the suprapygal and pygal plates, is widely accepted. One hypothesis assumes that costo-neural elements contain both the endo- and exoskeletal materials—in particular, dermal elements called the nu T1 osteoderm5,8,13–15,18,19. For shell acquisition, the osteoderms of T2 the ancestral animal was thus thought to have fused with the axial neu T3 skeletal elements (ribs and vertebrae) underneath. Osteoderms T4 are also seen in other tetrapods (for example, crocodilians and 3,21–23 cos T5 armadillos) , and the most conspicuous examples are found T6 in extinct animals, namely the mammalian glyptodontids, in T7 which exoskeletal elements form a thoracic shell. However, in T8 contrast to the completely immovable shell seen in the turtle, the T9 shells of the above-mentioned armoured tetrapods are not linked directly to the vertebral column or the rib cage, allowing free movement of the rib cage with the surrounding intercostal muscles. nu The second hypothesis assumes the endoskeletal origin of the neu d costo-neural carapace, maintaining that the costal and neural plates were simply acquired by modification of the axial skeleton and, therefore, that the major parts of the carapace were formed solely from the endoskeleton4,7,11. Lastly, in the third hypothesis, superficially translocated endoskeletal elements were thought to induce heterotopically exoskeletal osteogenesis of the carapace. Recent observations of the embryonic turtle suggest that heterotopic shifts of the ribs Figure 1 | Carapace of Chinese soft-shelled turtle Pelodiscus sinensis. occur during development: rib primordia translocated into the (a) Dorsal view. Scale bar, 1 cm. (b) Right half in ventral view. Red arrow dermis induce membranous ossification to differentiate flanges on indicates the costovertebral articulation. (c) Cleared and double-stained the craniocaudal aspects of the rib shafts and thus complete the juvenile specimen. Scale bar, 5 mm. (d) Enlarged image of the immature costal plate. The superficial shift of the ribs, initially arising costal plates in c. Box in c indicates the position of d. Scale bar, 2 mm. cos, endochondrally, is thought to cause a new tissue interaction in costal plate; T1–T9, thoracic vertebra numbers; neu, neural plate; nu, nuchal. the new location (that is, the dermis)16,17. epd drm drm icm sdc sdc icn pos icm St 17 St 18 St 21 ost drm pob drm pos btr sdc sdc St 23 St 27 Figure 2 | Development of the costal plate in the turtle. Hematoxylin-eosin-Alcian-blue-stained cross-sections of the ribs of the Chinese soft-shelled turtle (Pelodiscus sinensis). (a) Stage 17. Scale bar, 100 mm. (b) Stage 18. Scale bar, 100 mm. (c) Stage 21. Scale bar, 100 mm. (d) Stage 23. Scale bar, 100 mm. Arrows indicate direction of expansion of the periosteum (pos). (e) Stage 27. Scale bar, 100 mm. btr, bony trabecula; drm, dermis; epd, epidermis; icm, intercostal muscle; icn, intercostal nerve; ost, osteoblast; pob, periosteal bone collar; pos, periosteum; sdc, subdermal connective tissue. 2 NATURE COMMUNICATIONS | 4:2107 | DOI: 10.1038/ncomms3107 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3107 ARTICLE Here through a comparative developmental analysis, we demonstrate that the costal and neural plates are assigned to be pob hypertrophied ribs and vertebrae, respectively. These results ost indicate that the major part of the turtle carapace evolved solely pos by modification of the endoskeleton (that is, second hypothesis). Furthermore, in the fossil record, we recognized that precursors of carapace in some non-turtle diapsid reptiles developed also as an endoskeleton. The ribs of these reptiles are not only expanded in shape, as seen for examples, in extant anteaters24 or a Permian reptile Eunotosaurus25, but also laterally (nearly horizontally) projected as in the turtles. The genetic basis relevant to the carapace evolution is, therefore, likely to have deeper history than icm the split of the turtle lineage. Results plc The embryonic development of costal plates. We observed the embryonic histology of the Chinese soft-shelled turtle (Pelodiscus sinensis) to elucidate the embryonic environment of the developing costo-neural plates, which has long been ambiguous5–7,10–12,16,18. We found that the development of the costo-neural plates proceeds within the connective tissue associated with the axial muscles, under the dermis (Fig. 2). Specifically, at stage 17, the intercostal muscles are found between ribs: these tissues are embedded in thin connective tissue (Fig. 2a). At stage 18, the dermis appears as mesenchyme with a matrix that is stained with btr Alcian-blue; it has a clear boundary with the underlying subdermal connective tissue and the ribs (Fig. 2b). At this icm stage, the intercostal muscles begin to degenerate, but the subdermal layer remains at subsequent stages especially near the ribs (Fig. 2b,c). Thus the ribs remain to be encapsulated in the subdermal cell mass (Fig. 2c, sdc) under the dermis at stage 21 plc ost and thereafter. In the following stages (Fig. 2d,e), the rib icm periosteum expands craniocaudally within the subdermal cell Figure 3 | Membranous bone formation of the rib in birds. Hematoxylin- mass. The intercostal muscles disappear, but the subdermal layer eosin-Alcian-blue-stained cross-section of the rib in a chicken (Gallus remains present (Fig. 2d,e, sdc). In the rib periosteum of stage 27 gallus) embryo at stage 40. (a) Delicate bony trabeculae (osteoids) at the embryos, the flanges of the costal plates appear as membranously insertion of the intercostal muscle (icm). Arrows indicate protrusion of ossified bony trabeculae extending from the craniocaudal aspects bony trabeculae. Scale bar, 100 mm. (b) Bony trabeculae at the insertion of of the bone collar of the ribs (Fig. 2e, btr). In the post-hatching the intercostal muscle (icm). Scale bar, 100 mm. btr, bony trabecula; plc, development, these trabeculae extend farther outward from the pleural cavity; pob, periosteal bone collar; pos, periosteum; ost, osteoblast. rib shaft to complete the costal plate (Supplementary Fig. S2). The neural plate expands along the surface of the intrinsic back muscles, outside of the dermis (Supplementary Fig. S3). quite different from that of the costo-neural plates. Collectively, We also found that the outward growth of bony trabeculae our comparative analyses of these embryonic developments with an expansion of the periosteum (Fig. 2d,e) is not a turtle- suggest that the turtle’s costo-neural plates are entirely of specific developmental pattern, but is comparable to rib endoskeletal origin (that is, the second hypothesis). development in the chicken (Fig. 3). In the stage 40 chicken embryo, as in P. sinensis at stage 23 and thereafter, the rib periosteum expands to form bony trabeculae that extend outward Turtle-type carapaces of some basal diapsids. The results of our from the periosteal bone collars at the insertions of the intercostal embryonic analyses explain the nature of the stem turtle26 muscles. Taken together, the initial development of the turtle’s Odontochelys semitestacea, in which the carapace does not form costal plate and morphogenesis of the avian rib follow the a
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